Resilient Moduli Behavior of Lime-Cement Treated Subgrade Soils

Ranjan K. Rout1, Pinit Ruttanapormakul1, Shashank Valluru2, Anand J. Puppala3 1Graduate Research Assistant, 3Professor, Department of Civil Engineering, The University of Texas at Arlington, Arlington, Texas 76019, Email: anand@uta.edu 2 Project Engineer, Professional Service Industries, Houston, Texas 77007

ABSTRACT This paper presents the behavior of resilient moduli properties of three

subgrade soils selected from north Texas. These soils were tested for resilient modulus in accordance with AASHTO T-307 method. Lime-cement stabilization was first attempted on the control soils, the treated soils were also subjected to the same tests to evaluate the effectiveness of treatments in enhancing their resilient characteristics. Resilient Moduli values of untreated soils varied from 30 to 50 MPa whereas the same for treated soils varied from 90 to 150 MPa. Results showed a significant increase in the resilient properties of soils with lime-cement stabilization method. Also, the Universal model was used to analyze the effects of confining and deviatoric stresses on both soils. Model parameters for both lime-cement treated and untreated soils are determined and tabulated. Influence of chemical treatments on the model constants are explained.

Keywords: Resilient Modulus, Chemical Treatment, Strain Hardening, Strain Softening INTRODUCTION AND BACKGROUND

The resilient modulus (MR) is one of the important engineering properties of soil, which is generally used to represent the relationship between stress and strain of subgrade soils under a repeated dynamic loading generated by traffic (Yang et al. 2008). The guide for design of pavement structures (AASHTO 1993) and the mechanistic-empirical pavement design guide (M-EPDG) (NCHRP 2004) have developed and recommended the use of MR of subgrade soils in pavements structural design and analysis (Mallela et al. 2004; Mohammad and Saadeh 2008; Nazzal and Mohammad 2010). Therefore, several studies have been performed to study the influence of MR on the pavement design and evaluate the accurate MR values that are representative of the field conditions (Puppala et al. 2003; Kim and Siddiki 2006; Nazzal and Mohammad 2010). The resilient modulus, MR is expressed in terms of the ratio of the maximum cyclic stress ( ) to the recoverable resilient strain ( ) in a repeated dynamic loading environment and this expression is presented as shown in the following equation:

MR = (1)

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Generally, the resilient modulus of subgrade soil can be estimated by conducting cyclic load triaxial laboratory tests on the representative soil samples. According to the mechanistic-empirical pavement design guide (M-EPDG) (NCHRP 2004), the characterization of resilient properties of unbound materials can be accomplished at three different levels of accuracy. For the level 1 input parameters, direct laboratory testing of resilient modulus is required. The MR values used in Level 2 design can be calculated from the correlation with other soil properties. The level 2 inputs can be used instead if the MR laboratory test data is unavailable. Finally, the level 3 inputs offer the lowest level of accuracy as the MR used in this level is selected based on the material classification or default values (Kim and Siddiki 2006; Velasquez et al. 2009). The level 1 or laboratory based measured properties are considered the most appropriate ones for MEPDG design.

For correlations, the resilient moduli properties are modeled on various formulations. Model constants are determined and then they are correlated with basic soil properties. Currently, a generalized three parameter (k1-k2-k3) following constitutive model is used for characterizing the laboratory determined MR (Witczak and Uzan, 1988): 1 (2)

Where MR = resilient modulus; = bulk stress; = octahedral shear stress; Pa = normalizing stress (atmospheric stress = 101.4 kPa); and k1, k2, and k3 = regression parameters. Design and performance of pavement structures are dependent on the engineering soil properties, especially the resilient modulus values of the subgrade soils. Therefore, certain soils which are weaker need to be treated with chemical additives in order to improve their engineering properties. The effects of the soil-improving agents such as lime, fly ash, cement, and enzymatic stabilizer on the properties of the soils have been studied by many researchers and from those studies, it can be concluded that the soils treated by lime and cement provide the higher enhancements in mechanical properties than other additives (Puppala et al. 2003; Mallela et al. 2004; Puppala et al. 2007; Mohammad and Saadeh 2008).

Recently, Okyay and Dias (2010) studied the mechanical properties of lime and cement treated soil. Goderville silty soil samples were collected and treated with different lime and cement contents and then cylindrical soil specimens were prepared for unconfined compressive strength, indirect tensile strength, and direct shear tests. Besides, the prismatic specimens of 4 4 16 cm for the bending tests were also prepared. The specimens were kept in aluminum covered plastic for curing and the tests were performed at 7, 28, 90, and 350 days after preparing. The testing showed that the quantity of lime and cement contents affect the mechanistic properties of treated soils. However, the strengths of all treated soil specimens are increased. Almost 85 % of the total strength was attained in all cement treated soils after 90 days of curing.

Mohammad and Saadeh (2008) also studied resilient moduli and permanent deformation properties of stabilized base and subbase materials. In the testing of

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subbase material, lime and cement agents were used in the treatments. Cylindrical specimens of 152.4 mm in diameter and 304.8 mm in height of untreated and treated subbase soils were prepared and tested in a repeated load triaxial unit. In the case of permanent deformation tests, field sites were prepared with the same treated soils and these sites were monitored for total deformations. Test results showed that both lime treated and cement treated subbase soil yielded higher resilient modulus than that found for untreated soils. Low deformations were also observed on the cement treated soil section.

Mallela et al. (2004) provided the estimation of resilient modulus of lime-stabilized layer for mechanistic-empirical pavement design. Three levels of accuracy for the determination of MR are also provided. For the level 1, the AASHTO T-307 test protocol or an equivalent test can be used for determining the value of MR. At input level 2, the unconfined compressive strength (qu) need to be known to estimate the MR values by the following equation:

MR = 0.124(qu) + 9.98 (3)

Where MR = Resilient modulus in ksi and qu = unconfined compressive strength in psi. For the level 3 inputs, the MR can be estimated from experience or historical records. The objective of the present research is to perform the cyclic load triaxial laboratory tests on untreated and combined lime-cement treated subsoils following AASHTO T-307 method for evaluating and comparing resilient moduli for the untreated and treated soils. The regression model coefficients of the constitutive model are also determined for both treated and untreated soils.

SAMPLING AND TESTING PROGRAM

For the present research, three soils from three different sites (A, C and E) were obtained from Dallas County, Texas. Basic soil testing was first conducted on these soils. Both unconfined compressive strength and resilient modulus were determined at 95% of maximum dry density and wet of optimum (OMC+3%) condition. Tests were conducted in accordance with the ASTM and AASHTO test methods. The basic soil testing and index parameters as well as unconfined compression strength parameters of these soils are presented in Table 1. Table 1. Basic Soil Parameters of Control Soils

Site Classification Liquid Limit (%)

Plastic Limit (%)

Passing #200 (%)

MDD (pcf)

OMC (%)

UCS (psi)

A CH 58 40 84.8 101.6 20.5 25 C CH 61 44 88.2 101.9 20.8 18.8 E CH 71 50 95.6 96.9 23.8 23.9

Note: MDD – Maximum Dry Density in pcf and OMC – Optimum Moisture Content in percent (%)

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In order to estimate the amount of stabilizer needed, lime stabilization was performed based on the ASTM D 6276 method (ASTM, 2006). Based on these tests, a pH of 12.4 or greater is achieved at an approximately dosage of 6% of dry weight of the soil. According to the local agency’s Pavement Design manual, the percentage of stabilizers usage should meet a compressive strength of 160 psi. In order to achieve a high UCS value, cement treatment was also considered. Hence, the final mix design resulted in 6% of lime and 3% of cement at which the treated soils exhibited the required UCS value. Once the mix design was completed, standard Proctor compaction tests were conducted and these results are included along with UCS property details in Table 2. Table 2. Compaction and UCS Properties for Treated Samples

Site By Weight MDD (pcf)

OMC (%)

After 7 day Curing

Lime (%)

Cement (%)

UCS psi

A 6 3 96.4 23.3 172.8 C 6 3 99 16.9 244.7 E 6 3 93.9 23 195.7

Note: MDD – Maximum Dry Density in pcf; OMC – Optimum Moisture Content in percent (%); UCS – Unconfined Compression Strength in psi

From Tables 1 and 2, the unconfined compressive strengths of the soils are improved with the lime-cement treatment and a comparison of the strengths is presented in Figure 1, which indicate substantial improvements in UCS property.

Figure 1. Comparison of Unconfined Compressive Strengths for Treated and Untreated Samples

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Resilient modulus testing was performed as per AASHTO T-307 procedure on the remolded samples (AASHTO, 2003). The laboratory specimens were compacted to an L/B ratio of 2 i.e., 142 mm in height and 71 mm in diameter. The soil was air-dried and mixed with the desired dosage of lime and cement. The soil specimens were prepared at +3% of the optimum moisture content as wet of optimum condition is preferred for chemically treated soil. After initial mixing of soil with lime and cement, the mixture was mellowed for at least 72 hours in a closed container maintained at room temperature. Five layers of equal amounts of the mix were prepared and used to compact the samples. Subsequently, the specimens were extruded using a hydraulic jack. The extruded specimens were trimmed and weighed to check if the desired density was achieved. For untreated samples before testing, the samples were wrapped in a plastic bag and stored in a humidity room for two days whereas the treated samples were stored for 7 days for curing. Testing was conducted on three replicate specimens from each site.

LABORATORY TESTING PROGRAM Resilient modulus testing was performed in accordance with the AASHTO

procedure T-307. All tests were performed using the Repeated Load Triaxial (RLT) equipment with a closed loop servo pneumatic loading system. Applied load and strain were measured by the transducers which convert mechanical moment into standard electronic signals via CDAS (Control and Data Acquisition System) and provide the output display on the computer. The deformation was measured externally with two spring loaded linear variable differential transducers (LVDT). Water was used as medium for applying confining pressure.

Figure 2. Resilient Modulus Testing Setup

During testing, the soil specimen in the triaxial chamber was subjected to a

repeated cyclic stress and static confining stress conditions. The test starts with a conditioning phase by applying a minimum of 500 repetitions of load equivalent to a maximum stress of 27.6 kPa and confining pressure of 41.4 kPa. This phase eliminates the effect of the interval between compaction and loading, and minimizes the imperfect contact between the sample cap and the test specimen. Fifteen

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sequences of 100 repetitions each with haversine-shaped loads were then applied on the specimen. Different levels of deviatoric and confining stress were applied to the specimen to measure the resilient modulus. The load pulse used in this study had a 0.1 sec load duration and 0.9 to 3.0 sec rest period.

RESULTS AND ANALYSIS Comparison of MR of untreated and treated subgrade soils

The average values of the resilient modulus for the last five cycles for each stress sequence were calculated and are presented below. Figures 3, 4 and 5 present the resilient moduli of both untreated and treated soils of sites A, C and E respectively.

Figure 3. Resilient Modulus of Untreated and Lime-Cement Treated Soils of Site A

Figure 4. Resilient Modulus of Untreated and Lime-Cement Treated Soils of Site C

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Figure 5. Resilient Modulus of Untreated and Lime-Cement Treated Soils of Site E From Figure 3, the percentage increase in the resilient modulus of the treated

specimens with respect to untreated specimens of site A ranged from 60% to 160%. As depicted in Figure 4, the percentage increase in the resilient modulus of the treated specimens with respect to untreated specimens of site C ranged from 75% to 225%. The percentage increase in the resilient modulus of the treated specimens with respect to untreated specimens of site E ranged from 80% to 230% (Figure 5).

As expected, the resilient moduli values of treated soil are much higher than the untreated soil. It is observed that in untreated subgrade soil, at all three confining pressures, an increase in deviatoric stress resulted in a decrease of the resilient moduli values. This behavior is known as strain softening type and is expected for clayey soils. All three site soils exhibited this behavior in untreated condition. On the other hand, the treated soils exhibited an increase in resilient modulus with an increase in the deviatoric stress. This strain hardening behavior is expected for granular materials. This indicates that the treated soils exhibit non-plastic granular material behavior and their moduli results also show a dependency on the confining stresses.

These figures show that the confining pressure induce higher resilient moduli for chemical treated soils. Thus stabilizing the subgrade with lime and cement provides better foundation support than an untreated subgrade foundation.

Universal Model The resilient modulus behavior of subgrade soils are typically characterized using the Universal model, as shown previously in equation (2), and the laboratory resilient modulus values. In this study, statistical analysis was conducted to determine the regression coefficients (k1, k2, k3) for the soils. The regression coefficients of untreated and treated samples are tabulated in Table 3 and 4 respectively. From the results presented, the following observations can be made:

• The parameter of the model correlation k1, varied from 571 to 764 for untreated samples, whereas for treated samples, the same constant ranged

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from 1,015 to 1,202. The highest values were obtained for Site A in both untreated and treated conditions.

• The k2 parameter for untreated specimens varied from 0.23 to 0.31, whereas for treated specimens ranged from 0.42 to 0.58.

• The k3 parameter, on the other hand, varies from -2.53 to -2 for untreated samples and, whereas a constant value of -0.27 was obtained for treated samples.

Table 3. Results of Multiple Regression Analysis of Untreated Soil Specimens

Site Classification k1 k2 k3 R2 A CH 763.5 0.26 -2 0.99 C CH 571.8 0.31 -2.53 0.98 E CH 577.4 0.23 -2.14 0.97

Table 4. Results of Multiple Regression Analysis of Treated Soil Specimens Site Classification k1 k2 k3 R2 A CH 1201.7 0.42 -0.27 0.86 C CH 1015.5 0.58 -0.27 0.97 E CH 1126.6 0.56 -0.27 0.97

STRUCTURAL COEFFICIENTS

In the AASHTO flexible pavement design as noted in 1996 AASHTO design guide, the structural coefficients for each pavement layer (surface, base and subbase) are needed. These coefficients are typically calculated from each layers resilient or elastic moduli property by using the correlations given in the 1996 AASHTO flexible pavement design guide (Puppala et al. 2011). In this study, the equation recommended for determining the structural coefficient of treated soils (a3) is used as the present soils are primarily chemically treated. The equation is expressed below:

a 0.227 log MR 0.839 (4)

Where MR = resilient modulus in psi.

The average moduli calculated from the measured resilient moduli at five different deviator stresses for each confining pressure are substituted into the above equation to determine the structural coefficients. The calculated values of a3 of each site are represented in Figure 6. For the site A treated samples, the subbase layer coefficients ranged from 0.11 to 0.14, those for site C treated samples varied from 0.09 to 0.13; and those for site E treated samples ranged from 0.10 to 0.14.

SUMMARY AND CONCLUSIONS The present research summarized the resilient moduli of lime-cement treated

and untreated clayey soils from three sites. The comparison of resilient modulus (MR) of untreated and treated subgrade soils was made along with a regression analysis conducted on the measured moduli data for determining the values of the regression

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model parameters (k1, k2, and k3). Also, the structural coefficients of subbase layer of the treated soils were calculated from the average measured MR values. Based on the results and comparisons, the following conclusions are achieved:

Figure 6. Interpreted Structural Coefficients When the Treated Soils Serve as

Subbases

• For the combined treatment design, a lime dosage and cement dosage of 3% are recommended as all three site soils under these treatment levels exhibit UCS value well above 160 psi.

• As expected, the increase of resilient moduli (MR) was achieved in the lime/cement treated soils and the ranges of percent increase when compared to untreated soil moduli were 60% to 160%, 75% to 225%, and 80% to 230% for the soil specimens from site A, C, and E, respectively.

• The regression analysis provided excellent fit of the experimental data with high coefficients of determination values. The k1 and k2 parameters of treated soil specimens varied from 1,015 to 1,202 and 0.42 to 0.58 respectively whereas for k3 a constant value of -0.27 was obtained. When compared to untreated soils, the regression coefficient results obtained for treated soils are higher. This is because of the lime and cement treatment which resulted in enhancements of moduli.

• The subbase layer coefficients for the treated soils of site A ranged from 0.11 to 0.14, those for site C varied from 0.09 to 0.13 and those for site E ranged from 0.10 to 0.14.

REFERENCES AASHTO (1993). AASHTO guide for design of pavement structures, American

Association of State Highway and Transportation Officials, Washington, D.C.

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AASHTO (2003). AASHTO T 307-99 Standard method of test for determining the resilient modulus of soils and aggregate materials, American Association of State Highway and Transportation Officials, Washington, D.C.

ASTM (2006). D6276-99a Standard test method for using pH to estimate the soil-lime proportion requirement for soil stabilization, ASTM international, PA, United States.

Kim, D. and Siddiki, N. Z. (2006). “Simplification of resilient modulus testing for subgrades: final report.” Joint Transportation Research Program, West Lafayette, IN.

Mallela, J., Quintus, H. V., and Smith, K. L. (2004). “Consideration of lime-stabilized layers in mechanistic-empirical pavement design.” Report submitted to The National Lime Association. Champaign, IL.

Mohammad, L. N. and Saadeh, S. (2008). “Performance evaluation of stabilized base and subbase material.” Geocongress 2008: Geosustainability and Geohazard Mitigation, 1073-1080.

NCHRP (2004). Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Part2; Design Inputs, NCHRP 1-37A, Final Report, March 2004. National Cooperative Highway Research Program (NCHRP).

Nazzal, M. D. and Mohammad, L. N. (2010). “Estimation of resilient modulus of subgrade soils for design of pavement structures.” Journal of Materials in Civil Engineering., ASCE/July 2010, 726-734.

Okyay, U. S. and Dias, D. (2010). “Use of lime and cement treated soils as pile supported load transfer platform.” Engineering Geology, 114(2010), 34-44.

Puppala, A. J., Wattanasanticharoen, E., Hoyos, L. (2003). “Ranking of four chemical and mechanical stabilization methods used to stabilize low volume road subgrades in Texas.” Journal of the Transportation Research Board, National Academy of Science, Transportation Research Board, TRR No. 1819, Washington, DC, 63-71.

Puppala, A. J., Ramakrishna, A. and Hoyos, L.R. (2003). “Resilient moduli of treated clays from repeated load triaxial test.” Journal of the Transportation Research Board, National Academy of Science, Transportation Research Board, TRR No. 1821, Washington, DC, 68-74.

Puppala, A. J., Hoyos, L.R. and Potturi, A.K. (2011). “Resilient moduli response of moderately cement-treated reclaimed asphalt pavement aggregates.” Journal of Materials in Civil Engineering., ASCE/July 2011, 1-8.

Velasquez, R., Hoegh, K., Yut, I., Funk, N., Cochran, G., Marasteanu, M. and Khazanovich, L. (2006). “Implementation of the MEPDG for new and rehabilitated pavement structures for design of concrete and asphalt pavements in Minnesota: final report.” Department of Civil Engineering University of Minnesota, Minneapolis, Minnesota.

Witczak, M. W. and Uzan, J. (1988). The Universal Airport Design System, Report I of IV: Granular Material Characterization, Department of Civil Engineering, University of Maryland, College Park.

Yang, S., Lin, H., Kung J. H. S. and Huang, W. (2008). “Suction-controlled laboratory test on resilient modulus of unsaturated compacted subgrade soils.” Journal of Geotechnical and Geoenvironmental Engineering, Sep., 2008.

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